This quotation shows Lady Macbeth taunting her husband with cowardice and unmanliness if he goes back on his plans to kill Duncan and take the throne. Macbeth wants to murder Duncan, and is not afraid of divine sanctions, but as he lays out clearly in his speech at the beginning of Act I, Scene 7, there are many practical considerations that seem to make the murder a bad idea even when considered in an impartial, amoral light. In this scene, Lady Macbeth comes upon him as he is inclined to call the whole thing off,
We will proceed no further in this business:
He hath honor'd me of late, and I have bought,
Golden opinions from all sorts of people,
Which would be worn now in their newest gloss,
Not cast aside so soon.
Lady Macbeth is furious at what she sees as a great chance missed, and she taunts and bullies her husband until he changes his mind again, not only charging him with inconsistency but putting their relationship on the line:
Was the hope drunk
Wherein you dress'd yourself? Hath it slept since?
And wakes it now, to look so green and pale
At what it did so freely? From this time
Such I account thy love.
and directly accusing him of unmanliness:
When you durst do it, then you were a man;
And, to be more than what you were, you would
Be so much more the man.
Thus, the significance of this quote is that it shows how the interaction between the Macbeths leads them on to commit crimes that either of them might have hesitated to contemplate if they were alone. It also shows the weakness of Macbeth's character, that he can be turned from his well-crafted arguments against the murder so easily.